Titanium dioxide particles of the rutile or anatase crystalline form possess a well-known characteristic ability to absorb light in the ultraviolet (UV) wavelength range, a process that generates metastable electron poor and electron rich regions within the particle structure. If the surfaces of said particles are not suitably modified, the interaction of water and/or oxygen with said regions results in the generation of highly reactive, oxygen atom-based radical species that can over time degrade the appearance and/or the physical properties of many of the thermoplastic polymer matrices into which the aforementioned particles might be incorporated (see D. Holtzen, P. Niedenzu, M. Diebold, “TiO2 Photochemistry and Color Applications”, Society of Plastics Engineers' 2001 Annual Technical Conference Proceedings). Put differently, unless suitably modified, the UV light induced photoactivity of titanium dioxide particles for the most part prevents their use for the manufacture of thermoplastic polymer derived articles that require a commercially useful level of photodurability. Common examples of thermoplastic polymer matrices that are typically affected by this problem include, but are not limited to, those based on polyethylene, polypropylene and polyvinyl chloride.
A surface modification technique that is most often utilized during the commercial production of titanium dioxide particles to effectively mitigate the aforementioned photoactivity involves the encapsulation of said particles in a layer of amorphous silica which is then followed by the deposition of crystalline alumina of boehmite or boehmite-like morphology. An example of such a particle encapsulation process is taught in U.S. Pat. No. 5,993,533. The amorphous silica portion of this type of particle treatment is typically present at levels that range from about 1 wt % to about 10 wt % (total particle basis) while the crystalline alumina portion of said treatment is typically present at levels that range from about 1 wt % to about 5 wt % (total particle basis).
Another surface modification technique that can be employed to significantly mitigate the undesirable photoactivity of titanium dioxide particles involves encapsulating them in a layer of only amorphous alumina. An example of such a particle encapsulation process is taught in Example 1 of U.S. Pat. No. 4,460,655. In this process, fluoride ion, typically present at levels that range from about 0.05 wt % to 2 wt % (total particle basis), is used to disrupt the crystallinity of the alumina, typically present at levels that range from about 1 wt % to about 8 wt % (total particle basis), as the latter is being deposited onto the titanium dioxide particles. Note that other ions that possess an affinity for alumina such as, for example, citrate, phosphate or sulfate can be substituted in comparable amounts, either individually or in combination, for the fluoride ion in this process.
A significant disadvantage of the amorphous silica/crystalline alumina and the amorphous alumina-only particle encapsulation strategies is that the resulting particle encapsulations are prone to water retention and/or latent water generation. These tendencies can lead to the release of water vapor when the so-encapsulated titanium dioxide particles are incorporated into thermoplastic polymer derived articles using elevated temperatures. Such a release can unfortunately result in the formation of commercially unacceptable defects in the article that is being produced. When the article that is being produced is in thin film form, undesirable thin spots and/or holes can be produced in the film as it is being extruded, a process that is typically referred to as lacing.